The usual Semi-Continuous Deodoriser (SCD) is batch-operated, where oil batches are dumped from tray to tray in a tower construction having typically 8-11 trays. The main function of the deodoriser is taken place in 2 or 3 trays, where a stripping agent is brought into intimate contact with the oil by the use of Mammoth pumps or other gas-liquid contacting devices, thereby stripping off volatile components. The stripping agent is preferably steam, but can also be nitrogen, carbon dioxide or other inert gases, or combinations thereof.
The Mammoth pump is a further development of simpler contacting devices like a distribution pipe arrangement, i.e. straight pipe or ring shaped, for even distribution of the stripping agent in the oil phase by injection through a number of small holes/orifices. The function of the Mammoth pumps is that a stripping agent such as steam or vapour is injected in the bottom of the vertical or slightly angled pipes of the Mammoth pumps resulting in a Venturi-nozzle effect, thus transferring the oil from tray-bottom to above the liquid surface. The pump principle is also denominated a gas-lift pump.
The spray-effect and the liquid motion from a pressure related to the liquid height in the tray to the surface head space pressure enables the volatiles to be transferred into the gas phase and thus removed from the oil. The residence time provokes at the same time desired thermal decomposition of various components. To complete a certain stripping action, Mammoth pump operated trays require a long holding time compared to packed columns. If considering a typical physical refining task the required Mammoth tray holding time might be one hour, while a structured packing stripper can obtain same stripping efficacy in a few minutes.
The component types which are separated from the main product in deodorisation are often called “volatiles”, referring to their vapour pressures being considerably higher than the triglyceride oil itself and thus prone to vaporise when subjecting the mixture to high temperature and vacuum. Typical volatiles are decomposition products of low molecular weight such as short chain alkanes or aldehydes, as well as Free Fatty Acids (FFA), of which the latter type is often used as marker for the goodness of the deodorisation efficacy.
Many continuous deodoriser designs on market to day have the stripping action enhanced by contacting the oil and steam in true counter-current mode while the oil formed a thin film on a structured packing. The counter-current contact reduces the stripping steam consumption which due to the application of vacuum constitutes a significant part of the total running costs.
However, the continuous deodoriser designs address the large scale bulk operators and are not suitable for discontinuous operations due to high batch-to-batch cross-contamination and loss of production time at product change.
There is a steady need for flexible deodorisers to process specialty and tailor-made mixtures and thus a need for deodorisers which can process smaller batches without significant cross-contamination or time delay.
The availability of better crude oil qualities enables refining by the so called physical refining route, which offers lower operating costs than the alternative refining by alkali-neutralisation but requires higher stripping efficacy.
Also the interests in limiting the formation of undesired by-products such as trans-fatty acids etc are calling for improved stripping efficacy in semi-continuous deodorisers, where the stripping action traditionally are provided by Mammoth pumps.
However, the required steam consumption to conduct a certain separation task is by nature governed by the flow contact, and the cross-flow contact obtained in Mammoth pumps can never compete with the counter-current thin film packing in respect to utilisation of the applied stripping agent.